Without limiting the scope of the invention, its background is described in connection with npn bipolar transistors, as an example. The descriptions of current polarities herein would be reversed for pnp bipolar transistors.
A high-efficiency radio-frequency (RF) power amplifier design seeks to maximize the ratio of RF signal-power output to DC-power input. High-efficiency RF power amplifiers are often operated at conduction angles of 180 degrees or less (Class-B or Class-C). The conduction angle refers to the portion of one period of a sinusoidal input signal over which the transistor is "on", or conducting. A full period of the input signal contains 360 degrees.
If an amplifier transistor is biased such that it is conducting with no input signal, then for very small signals the conduction angle will be 360 degrees. Because the collector current symmetrically increases and decreases during successive half-cycles of the input signal, the average collector current remains constant. This is known as Class-A operation, with characteristics as illustrated in FIGS. 1a and 1b.
As the amplitude of the input signal is increased, a point is reached where the negative excursions of collector current are equal in magnitude to the no-signal collector current. Any further increase in input-signal amplitude forces the positive excursions (relative to the no-signal collector current) of current to become larger than the negative excursions (relative to the no-signal collector current). This is because the negative excursion of collector current for an npn transistor cannot be less than zero (sourced from ground). The average collector current is then higher than the no-signal collector current. This is known as Class-AB operation, with characteristics as illustrated in FIGS. 2a and 2b.
If the transistor is biased such that the no-signal collector current is nominally zero, but the transistor is on the verge of conducting, then one half-cycle of input signal will result in conduction and one half-cycle will not. The conduction angle is 180 degrees. This is known as Class-B operation, with characteristics as illustrated in FIGS. 3a and 3b. Average collector current will increase as input-signal amplitude increases.
If the transistor is biased such that the no-signal collector current is zero, and the current remains zero until the input-signal amplitude exceeds a finite threshold value, then the conduction angle is less than 180 degrees. This is known as Class-C operation, with characteristics as illustrated in FIGS. 4a and 4b. Average collector current is zero when the input-signal amplitude is below the threshold. Average collector current increases with increasing input-signal amplitude when the threshold is exceeded.
Of the operating modes discussed, Class-A operation generally results in the highest gain and lowest efficiency, while Class-C operation gives the lowest gain and highest efficiency. Class-AB and Class-B gain and efficiency characteristics are between these extremes.
Bipolar transistors contain no self-limiting mechanism for collector current. Instead, collector current generally continues to increase as input-signal levels increase until it reaches a maximum value determined by the ratio of available collector-supply voltage to collector-load impedance. Permanent device damage often occurs before any combination of external factors limits operating current. For example, the collector-emitter voltage at which avalanche breakdown occurs in some bipolar transistors decreases with increasing collector currents. If collector current increases (perhaps due to a lower-than-intended load impedance or higher-than-intended input-signal amplitude), then otherwise-safe collector-emitter voltages might lead to avalanche breakdown and transistor damage. When an amplifier circuit is designed using a bipolar transistor, one or more factors (such as input-signal level, collector-emitter voltage, or load impedance) may be controlled by design to restrict collector current to a safe level under normal operating conditions.